There are many advantages to variable valve timing, including improved efficiency, power and emissions. In cam-based engines, variable valve timing is commonly achieved by varying the relative angle between the crankshaft and camshaft. Hydraulically-actuated cam phasers may be used to provide the variation in the relative angle.
In such cam phaser devices, a plurality of advance chambers and retard chambers are defined between a rotor and a stator. The rotor is coupled to the camshaft, and the stator is coupled to the crankshaft via a timing belt or chain. A hydraulic valve system is employed to control relative hydraulic pressure between the advance and retard chambers. To advance cam timing, hydraulic pressure is increased in the advance chambers relative to the retard chambers, thereby producing a relative rotation between the rotor and stator. Conversely, timing is retarded by increasing pressure in the retard chambers relative to the advance chambers. A given timing is maintained by keeping the pressures within the advance and retard chambers substantially equal.
While providing effective variable valve operation, many cam timing phasers produce significant noise. For example, when maximum retarded or maximum advanced timing is commanded, the hydraulic forces can cause the rotor to impact the stator at a significant velocity. In addition, when the rotor is close to the stator (e.g., nearly fully advanced or retarded), cam torsional effects can cause the rotor to forcefully impact the stator. This can result in noise, vibration and harshness (NVH) levels high enough to cause operator dissatisfaction.
Accordingly, the present description provides for variable cam timing phaser having a stator and a rotor. The stator has a plurality of inwardly-extending stator lobes, and the rotor has a plurality of outwardly-extending rotor lobes. The rotor is rotatably disposed within the stator so that the rotor lobes interleave with the stator lobes to form a first timing chamber and a second timing chamber between each of the stator lobes.
According to one example, the phaser further includes a valve, and where upon operation of the valve to selectively couple the second timing chambers to a hydraulic fluid supply and the first timing chambers to a hydraulic fluid sink, the rotor is caused to rotate toward a terminal position, in which at least one of the first timing chambers is at least partially sealed off from the hydraulic fluid sink, thereby producing a tendency toward pressure equalization between the first timing chambers and the second timing chambers.
According to another example, the phaser further has a plurality of hydraulic fluid orifices. One such orifice is associated with each of the first timing chambers for permitting hydraulic fluid to fill and drain from each of the first timing chambers. The orifices are positioned so that when the stator and rotor are in a first relative rotational position, each of the orifices is fluidly coupled with its associated first timing chamber. When the stator and rotor are in a second relative rotational position, at least one of the orifices is sealed off from its associated first timing chamber.
In certain settings, the exemplary embodiments described herein provide the advantages of variable cam timing, while minimizing or eliminating the undesirable NVH levels produced by prior variable cam timing systems.